KR940010644B1 - Measuring method of semiconductor energy gap and apparatus the same - Google Patents

Abstract

The apparatus for screening energy gap of semiconductor material using an image processing system comprises a polychromator (3) for projecting the spectrum of light on a sample (4) through a lens (2), optical filters (5) for passing the image of transmission spectrum according to specific wave lengths, an image device (6) for converting image made of the corresponding filter's wave length to analog electrical signal, an image processing system (7) for converting digital signal and memorizing it an energy gap access & display device (8) for deciding function relation between pixel coordinates value and wave length.

Description

Energy gap measurement method and device in peninsula

1 is a graph illustrating energy gap induction on an alpha (α) graph, (a) is a graph showing a case of direct transition, and (b) is a graph showing a case of indirect transition.

2 is a characteristic diagram of a transmission spectrum of a semiconductor having a direct gap.

3 shows a schematic interpretation of the principle applied to the present invention.

4 is a diagram showing an embodiment of the present invention.

5 is a flowchart of the present invention for an energy gap measurement method.

* Explanation of symbols for main parts of the drawings

1: light source 2: lens

3: polychromator 4: sample

5: optical filter 6: image reading device

7: Image signal processing device 8: Energy gap search and display (Energy Gap Access & Display)

9: personal computer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring an energy gap (Eg) of a semiconductor, and more specifically, to assess an energy gap of a semiconductor material using an image processing system. The present invention relates to a method and an apparatus thereof.

Until now, mainly on the basis of the theory described below, the most important electrical properties of semiconductor materials are the use of a spectrophotometer that continuously changes the wavelength of a light source and scans the specimen to obtain a transmission spectrum. The magnitude of the energy gap is derived. Such optical means are known as the most important means for measuring the energy gap size of a semiconductor. When the photon of energy h h near the size of the energy gap Eg of the semiconductor material is shot, electrons are emitted from the conduction band of the semiconductor to the valence band and the photons are absorbed. This optical hop mechanism is called fundamental absorption. As such, the energy gap Eg of the semiconductor material can be measured from photon induced electronic transitions between bands due to photons having a predetermined band of energy. The Dubrowskii equation for optical transmission in semiconductors is as follows.

Where α is an absorption coefficient,

d: thickness of semiconductor specimen

R: Reflectance

A + T + R = 1

Equation (1) may be simply expressed by the following MOSS equation when the amount of optical absorption is relatively large.

T = (1-R) ² exp- ^{(? D)} ... … … … … … … … … … … … … … … … … … … ②

The absorption coefficient α at the fundamental absorption edge (band to band transition) is expressed as follows for the direct transition and the indirect transition, respectively.

* In case of direct transition

α = A (hν-Egd) ^½ : hν> Egd

= 0: hv≤Egd... … … … … … … … … … … … … … … … ③

* Indirect transition

α = A (hν-E _θ- Egi) ² + C (hν-E _θ -Egi) ² : hν> Egi-E _θ

= 0: h v ≤ Egi-E _θ . … … ④

In Equations ③ and ④ above, Egd represents direct energy gap, Egi represents indirect energy gap, and E _θ represents phonon energy, respectively, and the first term B (hν-E _θ -Egi) ² of Equation ④ Means that the phonon is emitted, and the second term C (hν + E _θ −Egi) ² indicates that the phonon has been absorbed. Therefore, Egd and Egi derive the relationship of α ² = f (hν) and α ^1/2 = f (hν) on the graph using Equations ③ and ④ as shown in FIG. Here, α is obtained from the output of the spectrophotometer, that is, the transmitted light and the reflected light (T, R) when the thickness of the specimen is known through Equation ① or Equation ②. In Equation (3), in the case of direct transition, α = 0 at photon energy below the energy gap Eg, so the transmission spectrum is the second in the case of a semiconductor in which impurities are not embedded with the energy gap Eg as the threshold energy. As shown in the figure, the energy gap Egd can be obtained directly from this spectrum. That is, in FIG. 2, the starting point of the transmission points to the energy gap Eg. Most compound semiconductors have direct energy gap characteristics, which makes this method applicable.

An object of the present invention is an energy gap in which a predetermined range of light sources are transmitted through a specimen and imaged in response thereto, and then the energy gap (Eg) value is directly read from the image using a digital image processing system. To provide a measurement system.

3 is a view for explaining the principle of the energy gap measurement method according to the present invention, the transmission spectrum characteristics obtained by transmitting a light source having a predetermined wavelength (λ) band centering on the energy gap (Eg) through the specimen as an image The energy gap value is read from the image by the image processing system. In FIG. 3, it can be seen that as the absorption coefficient α increases, that is, as the absorption of light increases, the color of the image becomes darker. In other words, the image becomes darker toward λ ₁ .

4 shows an embodiment of an energy gap measuring apparatus according to the present invention, in which reference numeral 1 denotes a light source, 2 denotes a lens for condensing, and 3 denotes an output slit of a general-purpose monochromator. 5 is an optical filter for passing only a specific wavelength (λ _p ) necessary for setting image coordinates, and 6 is an image reading device such as a silicon Vidicon black or a charge coupled device (CCD) camera. image reading device, 7 is a digital image signal processing device such as PCVISION FRAME GRABBER from imaging techique, and 8 is an energy gap that establishes a functional relationship between an image pixel value and a wavelength (λ). A search and display device 9 is a personal computer that performs arithmetic and control functions related to measurement. The image signal processing device 7 and the energy gap search and display device 8 may be installed in the microcomputer 9. Here, we will explain how to set the χ coordinate value (λ) of the image.

First, the light of a certain wavelength band λ centered on the energy gap Eg is irradiated onto the specimen 4 to image the emitted spectrum. Therefore, the actual wavelength values of the band must be identified. do. In other words, the relationship λ = f (χ) between the wavelength λ and the coordinate value χ in the image displayed on the monitor connected to the monitor or the image signal processing device 7 of the personal computer 9 should be obtained. The method is characterized in that the specific wavelengths behind the specimen 4 shown in the embodiment of FIG. Place a plurality of optical filters 5, which are λpn, in sequence and for each pixel value χp1, χp2,... for these wavelengths in the monitor. Find the coordinates of χpn.

That is, each optical filter passes only the light of the wavelength λp and only the spectral image thereof appears on the monitor, so the χ coordinate of the image indicates λp. More specifically with reference to FIG. 3, the specimens are irradiated with infrared light as a light source in a section (typically 0.7-1.5 μm wavelength) in which a spectrum (each rainbow color) having different wavelengths is continuous. However, at this time, since the wavelength value of each spectrum is unknown, optical filters that pass only the measurement wavelength are used to know this.

Therefore, when imaging the light passing through the specimen, place a plurality of light filters (eg, red, orange, yellow, green, blue, indigo, violet) from the left side of the specimen between the specimen and the image reader. In this case, an image of light wavelengths corresponding to red will appear on the leftmost side of the monitor, and an image of light corresponding to violet will appear on the rightmost side of the monitor. In this way, if an optical filter with a green wavelength is placed between the specimen and the image reading device, an image of only green light (wavelength) appears on the monitor, so the χ coordinate value on the monitor of the image corresponds to green It means the wavelength. In this way, the optical filter is used as a means for deriving the relationship of λ = f (χ).

Here, if the wavelength change in the spectrum is linear, only two filters can easily induce a relationship of λ = f (χ). That is, since the optical filter passes only light of a specific wavelength λp, if a predetermined optical filter (passing only the wavelength of λp1) is placed at the position of the specimen, the image appearing on the monitor image is applied to the light λp1 corresponding to the filter. Will be an image, and the X coordinate of this image will be a position indicating the wavelength lambda p1. In this way, one point A (χ ₁ , λp1) is obtained, and another point B (χ _2, λ _p2 ) is found in the same way using an optical filter passing only λ _P2. From λ = f (χ) can be obtained. Now, the energy gap measuring method will be described with reference to FIG. 5 below.

5 is a flowchart for measuring an energy gap, which first sets an energy gap pixel value Xgap on an energy gap Eg measurement program in advance (step 1). In other words, semiconductors generally contain impurities, which form impurity levels in the energy gap (Eg), resulting in impurity hops before the fundamental absorption occurs. It occurs from the photon energy lower than Eg). Therefore, in consideration of this effect and the gray level of the background image according to the intensity of the light source used in the optical system, the appropriate pixel value Xgap corresponding to the energy gap (Eg) is preliminarily determined through the reference specimen which is previously characterized. Must be set (step 1).

Subsequently, as described above, the predetermined optical filters 5 are sequentially placed behind the specimen 4, and are set in step 1 through the X coordinate of the wavelength value of the corresponding filter 5 in the image displayed on the monitor 8. The transfer function of λ = f (χ) for one optical system is obtained through an optical filter, and then the specimen 4 is placed to perform imaging (step 2), and the image input device 6 The live image is stored in the frame memory of the video signal processing apparatus 7. Subsequently, pixel values are scanned from left to right (or right to left) along the χ axis in the image on the monitor (step 3). The pixel value X thus scanned is compared with the energy gap pixel value Xgap (step 4). If the two values coincide, read the χ coordinate of the pixel to get λgap using the algorithm of λ = f (χ). The gap is changed in units of [eV] in the relationship of (step 5). Conversely, if the two values do not match, the optical system is adjusted or the sample is repositioned to obtain an analytical image (step 4-1). The value of the energy gap Eg changed in units of [eV] is displayed on the screen and the energy gap Eg measurement program is terminated (step 6).

As described above, instead of using a conventional spectrophotometer which continuously changes the wavelength of the light source and scans the specimen to obtain a transmission spectrum, the response is imaged by transmitting a band of light sources through the specimen in a batch. By reading the energy gap values directly using a digital image processing system having a relatively simple configuration, the energy gap can be measured more accurately and faster.

Claims (3)

Projecting the reference specimen into a light beam having a predetermined wavelength band, converting the χ coordinate of the transmission spectrum image into a wavelength value through optical filters, and analyzing the characteristics of the reference specimen to set an energy gap pixel value (Xgap) And obtaining a transfer function λ = f (χ) between each pixel value (χ) constituting the transmission spectrum image and the wavelength (λ) of the pixel, and then positioning the specimen to perform imaging of the spectrum. After storing the live screen obtained through the imaging, scanning each pixel value (χ) along the χ axis of the image, and sequentially comparing the respective pixel value (χ) and the energy gap pixel value (Xgap) And reading the χ coordinates of the matching pixels to obtain a wavelength λgap and converting the energy into units [eV] to obtain an energy gap (Eg). The pixel value corresponding to the energy gap is pre-selected, and after scanning each pixel value of the χ coordinate from the left / right side to the right / left direction of the image, the two values are compared and matched. A method for measuring energy gaps in semiconductors, characterized in that the coordinates are read as an energy gap value. A device for measuring the energy gap (Eg) of a semiconductor by an optical method, comprising: a personal computer (9) which performs various operations and control functions related to the measurement of the energy gap (Eg) and emitted from the light source (1) A lens 2 for collecting light, a multicoloring means 3 for irradiating the sample 4 with a spectrum of light provided through the lens 2, and light of a transmission spectrum image provided through the sample 4 A plurality of optical filter means (5) for passing each of the plurality of specific wavelengths (λ _{p1 to} λ _pn ) different from each other, and the plurality of optical filter means (5) generate an image of the wavelength of the filter. A video reading means 6 for converting into a signal, a video signal processing means 7 for converting and outputting an output signal from the video reading means into a digital signal, and setting a functional relationship between a coordinate value of a pixel and a wavelength Energy gap search and indicator means (8) Semiconductor energy gap measuring device comprising.